Switches in radio-frequency technology, for example, as described in the US patent specifications U.S. Pat. No. 6,225,874 B1 and U.S. Pat. No. 5,307,032, can be realized through a coupled line system. At these line systems, switching elements are disposed in each line. The switching elements are arranged exclusively at the inputs and outputs of the line system. With these switches, a low insertion loss of an electromagnetic signal to be transported is desirable for the respective switching path. Because of the always present parasitic inductances and capacitances of the switching elements, this low insertion loss can no longer be achieved for these switches in the case of very high frequencies—especially in the multiple-digit gigahertz range.
In this context, the selection of the respective switching path is implemented by means of a DC voltage or a DC current. However, in radio-frequency technology, it is desirable for the inputs, or respectively outputs, of a switch to be free of DC voltage and DC current. Furthermore, it should not be possible to vary the selection of the switching path through an external DC voltage source at the inputs respectively outputs of the line system. In order to achieve this, a coupling capacitor is inserted at the inputs and outputs of the switch. These coupling capacitors have a lower limit frequency determined by the capacitance value. If the switch is to be usable from a low bottom limit frequency up to a high top limit frequency, these coupling capacitors must be resonance free and must have a low insertion loss over this frequency range. With currently available coupling capacitors, this is not realizable. As a result of the coupling capacitor, the lower limit frequency cannot be zero, so that a DC voltage cannot be transmitted via such a switch.
Switching over the switching path is implemented by changing a DC control voltage respectively a DC control current. This causes voltage peaks at the inputs and outputs of the coupled line system. Such so-called video crosstalk can be very high and is undesirable.
In US patent specifications U.S. Pat. No. 6,225,874 B1 and U.S. Pat. No. 5,307,032, the switching elements used for switching the lines are disposed exclusively at the inputs and outputs of every line of the line system. Accordingly, a switching element is also disposed in the line which is connected to the output of the switch. This causes an increased video crosstalk which is undesirable. Additionally, wideband coupling capacitors, which cannot be realized with low insertion loss and in a resonance free manner, are necessary.
In order to achieve a low insertion loss of a signal to be transported via a coupled line system, a strong coupling between the lines is necessary at the line systems described in US patent specifications U.S. Pat. No. 6,225,874 B1 and U.S. Pat. No. 5,307,032. Coupled line systems with a strong coupling between the coupled lines are significantly more difficult to realize than coupled line systems with weak coupling between the coupled lines. As a result of the strong coupling, a high radio-frequency current flows through the switching elements which are switched to a low impedance. As a result of the power dissipation accordingly occurring in the switching elements, the maximal radio-frequency input power of such switches is severely limited. Because of the high radio-frequency current through the switching elements, the radio-frequency parameters of the switching elements are strongly modulated. This leads to nonlinear distortions which are undesirable.
Accordingly there is a need to provide a line system with coupled lines which overcomes the disadvantages identified above. In particular, the coupled line system should provide a low insertion loss and a low attenuation of the signal to be transported. In this context, the line system should be able to transport both DC-voltage signals, signals with low frequency and also signals with a very high frequency—especially in the multiple-digit gigahertz range. In particular, the line system according to the invention should be usable for the transportation of signals with a high radio-frequency power as well as low nonlinear distortions and low video crosstalk.